Steam boiler or the like having extended heat transfer surfaces



May 10, 1949. D. DALIN ETAL STEAM BOILER OR THE LIKE HAVING EXTENDED HEAT TRANSFER SURFACES 6 Sheets-Sheet 1 Filed 001:. 6, 1948 AIR HEATER ECONOMIZER SUPERHEATER STEAMING 91 .2. A (LA MONT TYPE BOILER) TOTAL CONVECTION SURFACES FOR SAME OILER USING PRESENT INVENTION STEMIING, SUPERHEATER AND ECONOMIZER SECTIONS CONVECTION SURFACES OF AIR H ATER CONVECTION SURFACES OF -VIVIVIVIVLI Susi/av Y H5919 D. DALIN EI'AL STEAM BOILER OR THE LIKE HAVING EXTENDED May 10, 1949.

HEAT TRANSFER SURFACES 6 Sheets-Sheet 2 Filed Oct. 6, 1948 .Uavzd .UaZzn E'us'iav IZHagE-y a 4 cw 6 Sheets-Sheet 3 a MM-M .Uavzd .UaZzz-z May 10, 1949.

Filed Oct. 6, i948 BOILER ALREADY IN SERVICE EFF/C/E/VC' Y 77 May 10, 1949. D. DALlN EI'AL 2,469,635

STEAM BOILER OR THE LIKE HAVING EXTENDED HEAT TRANSFER SURFACES Filed Oct. 6, 1948 6 Sheets-Sheet 5 FAMILY OF CUR 1053 FOR W/RE EL Ems-N73 0F 5 MM D/A. AND 7106MM2 muss 5507/0 ll ll Ii M Davzd .Ualm

325 FOR COPPER 571525? THE-59b) a7la=aAs aF/VAIVRAL II II May 10, 1949. D. DALIN ETAL STEAM BOILER OR THE LIKE HAVING EXTENDED HEAT TRANSFER SURFACES 6 Sheets-Sheet 6 Filed Oct. 6, 1948 COMBUSTION ZONE UUVII Patented May 10, 1949 STEAM BOILER OR THE LIKE HAVING Ex- TENDED HEAT TRANSFER SURFACES David Dalin, Ronninge, and Gustav V. Hagby, Ostertalje, Sweden, assignors to A/B Svenska Maskinverken, Sodertalje, Sweden, a corporation of Sweden Application October 6, 1948, Serial No. 53,004 In Sweden January 3, 1948 13 Claims. (Cl. 122-1) Our invention relates to extended surface heat exchangers such as used in steam boilers and the like for effecting indirect heat transfer between two media having diiferent surface conductances and contemplates a structure for this purpose which will be capable of withstanding continued subjection to gas temperatures higher than 1200 degrees centigrade. The invention is especially applicable to the convection surfaces of steam boilers. So utilized it effects as much as a ninety percent reduction in the amount of tubing required for the convection surfaces and reduces the space required by the convection surfaces to one-eighth that necessary in a La Mont type boiler, which is considered one of the most efficient boilers presently available.

Our invention also makes it possible to greatly increase the efficiency of boilers and furnaces now in service by the simple expedient of incorporating a relatively small unit constructed in accordance with this invention in the flue gas duct connecting the boiler with the stack; and by virtue of the sniall space they require, boiler units constructed in accordance with this invention are ideally suited to the utilization of otherwise wasted heat emanating from blast furnaces and other industrial heat sources.

By the same token the invention is especially important to marine boilers where its increased efficiency enables a higher steaming rate and consequently faster speeds, or conversely because of reduced fuel consumption and decreased weight, effects an increased cruising range.

Our invention also contemplates the improvement and reduction in overall size of oil preheaters and similar apparatus where condensing steam is frequently used as the heating medium, and where the designers of past apparatus for this purpose took no account of the difference in conductance values of oil and steam.

We have also found that the utilization of radiant heat, as in hot air furnaces, can be greatly improved by the application of this invention thereto.

Our invention recognizes the fact that by increasing the heat transfer surfaces exposed to the med um of lower surface conductance and proportioning and arranging them in accordance with certa n formulae. the rate of heat transfer of the medium of lower surface conductance can he b ought more nearly into balance with the rate of heat transfer of the medium of higher surface conductance.

Specifically our invention consists in the pro-. vision of extended surface in the form of rod-like heat transfer elements joined to the boiler fluid containing tubes or partition wall separating the two media and extending into and substantially across the zone containing the medium of lower surface conductance so as to be in direct heat transfer relation thereto while having indirect heat transfer relation via the wall with the medium of higher surface conductance. The objectives of the invention are achieved through proper proportioning and arrangement of these heat transfer elements.

It is recognized that numerous attempts have heretofore been made to improve heat exchange between two media through the provision of fins, rods and other elements carried by the tubes or other partition walls separating the media and extending into one orboth of the media, but as far as we know there has never been a teaching of how through theuse 'of extended surface the convection surfaces of a steam boiler of the La Mont type, for instance, could be reduced to such an extent as to require only oneeighth the space heretofore necessary. To the best of our knowledge the selection of the shape, proportions and geometrical arrangement of extended heat transfer surfaces has always proceeded upon a cut and try basis, and such empirical experimentation has been very costly. In contrast our invention is predicated'upon the discovery of certain demonstrated scientific facts concerning these factors and their significance, and as will appear more fully hereinafter. provides adequateinstructionsfor the design of apparatus capable of achieving the purposes of this invention.

In summation the objectives of our invention are:

(1) To provide a more eflicient indirect heat exchange between two media of diflerent surface conductances;

To provide heat exchange apparatus especially adapted for use as the convection surfaces of a steam boiler and by which the tubing required is reduced to one-tenth that heretofore necessary and the overall space required for the convection surfaces is reduced to approximately one-eighth that heretofore necessary;

(3) To provide means for increasing the efficiency of boilers and furnaces now in service;

(4) To reduce the size and increase the emciency of oil preheaters;

() To improve the utilization of radiant heat.

With the above and other objecm in view which will appear as the description proceeds, our invention resides in the novel apparatus substantially as hereinafter described and more particularly defined by the appended claims. it being understood that such changes in the precise embodiment of the hereindisclosed invention may be made as come within the scope of the claims.

The accompanying drawings illustrate several complete examples of the physical embodiment of the invention constructed according to the best modes so far devised for the practical application of the principles thereof, and in which:

Figure 1 is a view diagrammatically illustrating a La Mont type boiler having its convection surfaces constructed in accordance with our invention, the illustration of these surfaces, for the sake of clarity, being somewhat exaggerated and thus not truly in proportion with the rest of the apparatus; r

Figure 2 is a diagrammatic fllustration of a typical La Mont boiler showing how much of its total space requirements are devoted to the convection surfaces, and showing by comparison the vast reduction of this space effected by the present invention;

Figure 3 is a longitudinal sectional view upon an enlarged scale through the flue gas e of the boiler shown in Figure 1 and illustrating a section of the convection surfaces;

Figure 4 is a cross sectional view through Figure 3 on the plane of the line H; Figure 5 is a detail sectional view upon a still larger scale through two tubes of the convection surfaces shown in Figure 3 with the heat transfer elements attached thereto:

Figure 6 is a cross sectional view through Figure 5 on the plane of the line H showing the arrangement of the heat transfer elements with respect to each other and the gas flow thereover;

Figure 7 is a view partially in side elevation and partially in longitudinal section illustrating the application of this invention to an existing boiler installation to utilize the ordinarily wasted heat of the exhaust flue gases and thereby increase the efliciency of the boiler;

Figure 8 is a cross sectional view through Figure 7 on the plane of the line 8-0;

Figure 9 is a graph showing the relative elliciency of the boiler to which we applied the apparatus of Figures '7 and 8, before and after such application; s

Figure 10 is a graph explaining the symbol used herein to denote the conductivity efllciency of the individual heat transfer elements which collectively comprise the extended surface;

Figure 11 is a chart showing a set of curves we have plotted to facilitate determination of the maximum length of the individual heat transfer elements in accordance with our invention;

\ Figure 12 is a vertical sectional view through a hot air furnace illustrating the application of thk invention to the improvement of the utilization of radiant heat;

Figure 13 is a view partially in side elevation and partially in longitudinal section illustrating the application of this invention to an oil preheater; and

Figure 14 is a cross sectional view through Hgure 13 on the plane of the line ll-ll.

Before specifically considering several embodiments of the invention illustrated in the accompanying drawings, itis essential that the term surface conductance be fully understood. By this term is meant the ability of a medium to effect heat transfer between itself and a surface of a solid exposed thereto. It involves such considerations as the physical properties peculiar to the medium; the temperatures of the medium and the surfaces involved; the velocity of the medium if flowing or in motion; the shape and dimensions of the surface exposed to the medium; the dis-- position of the surface in the medium with respect to its direction of flow or motion; and its proximity to other surfaces or obstructions within the medium which would have an effect upon the manner in which the medium contacts the surface.

More specifically the term "surface conductance" as used herein (throughout the specification and claims) has the same meaning as the designation "alpha value" obtained by the follow- 8 formulae and correction we have found necessary:

For the staggered arrangement of the heat transfer elements providing the extended surface as shown in Figure 6 a For a rank and file arrangement of the heat transfer elements. i. e. where the elements are arranged one behind the other in side-by-side rows or flles- S. is the center-to-center distance between adjacent heat transfer elements in meters and measured at right angles to the direction of as flow S1 is the center-to-center distance between adjacent heat transfer elements in meters and measured in the direction of gas flow T is the absolute temperature of the gas=t,+273' V0 is the velocity of the gas in m./s. at 0 C.

d is the diameter of the heat transfer elements in meters While these formulae have been obtained from Dr. Ing. Alfred Schacks bool: Der Industrielle Warmeubergang (1940 edition), page 121, we

have found that the values obtained thereby are too high by thirty percent (30%) for the practical application in apparatus of the kind contemplated by this invention. Seventy percent of the values obtained by the application of the preceding formulae are correct. Hence, where the term "surface conductance is used herein it should be understood that we mean the value obtained by the given formulae corrected by a thirty percent (30%) reduction.

It should also be specifically noted that for the conditions which normally obtain in a steam boiler the surface conductance of steam or water,

7 i. e. the boiler fluid. is at least one hundred times greater than that of combustion gases; and that at 50 degrees C. and the same velocity the surface conductance of oil is only nine percent (9%) that of water.

From this it follows that no useful purpose is achieved by merely increasing the surface area of a wall separating water and hot combustion gases for instance, equally at both sides of the wall, for by virtue of its greater surface conductance the rate of heat transfer which obtains between the water and the surface in contact therewith is far greater than the rate of heat transfer that can be obtained between the hot gases and the same wall.

It also follows that if the heat transfer surfaces in heat transfer relation with the hot gases are increased and those surfaces are properly proportioned and disposed with respect to the gas flow, it is possible to increase the rate of heat transfer between the gases and the surfaces exposed thereto to the extent that said rate begins to balance the maximum obtainable rate of heat transfer between the water and the inside of the tube containing it. A mere haphazard collection of prongs or fins on the outside of the tube, however, does not suffice.

It should also be understood that practical considerations-for instance the excessively high temperatures to which apparatus of this character is subjected, the need for cleaning oif accumulations of soot and dirt from the surfaces, and the basic mechanical strength requirements-increase the difliculties which the problem presents.

Referring now particularly to the accompanying drawings and especially to Figures 1 to 6 inclusive, the numeral 5 designates the fire box or combustion zone of a La Mont type boiler the walls of which are lined with tubes 6 as is customary. Water is circulated through these tubes from a steam dome 'l by means of a pump 8. The combustion gases leave the fire box or combustion zone through a fiue gas duct 9 which leads to the stack (not shown).

Within the flue gas duct 9 are the convection surfaces ll] of the boiler, comprising a steaming section, a superheater section, an economizer and an air heater as indicated in Figure 1. The manner in which the several sections of the convection surfaces are connected in the system follows conventional practice. Though mentioned hereinbefore it is desired to again point out that for the sake of clarity the illustration of the convection surfaces in Figure 1 is somewhat exaggerated. Actually they would occupy less space than that illustrated.

The saving in space which our invention effects in a La Mont type boiler is graphically shown in Figure 2. Here a typical La Mont type boiler is diagrammatically illustrated with that portion thereof containing the convection surfaces and air heater cross hatched. In accordance with existing practice the flue gas duct is shown extending down along the back wall ll of the combustion zone, this arrangement having been considered most feasible from the standpoint of space savmg.

With the application of the present invention to the La Mont type boiler the great reduction in the space necessary to house the convection surfaces permits the flue gas duct to extend directly up from the top of the combustion zone as shown in Figure 1.

As shown in Figures 3 to 6 inclusive the amount of tubing [2 employed for the convection surfaces is materially less thanthat heretofore necessary. Actually the reduction amounts to ninety percent (90%) This follows from the fact that the primary heat transfer surfaces are not the tube walls but instead consist of extended surface provided by heat transfer elements l3 suitably Joined to the tubes in intimate heat transfer relation thereto.

The elements I3 havea small cross section compared to their length. Specifically the cross sectional area of the elements should be between 3 and 50 mm. and their cross sectional shape is preferably round though any other section may be used as long as the major transverse dimension does not exceed three times the minor transverse dimension. This assures desirable turbulent flow.

The minimum length of the elements, 1. e. of their conductance paths, should be not less than ten times the square root of their cross section, or to be more exact, the minimum length of the conductance path (measured in mm. or any other suitable unit of measurement) shall not be less than ten times the square root of the number expressing the cross sectional area of the element (in mm. or whatever other unit of measurement is employed to express the length of the path).

The optimum maximum lengthof the elements bears a definite relationship to the temperature to which the elements are subjected, the cross sectional area of the elements, the conductivity coefficient of the elements and the surface conductance of the gas, and can be determined with relative accuracy by means of the following formula in which the symbol signifies the heat transfer efliciency of the elements:

In further explanation of the heat transfer efficiency 11g, reference is made to the diagram shown in Figure 10. From the diagram it will be seen that m; represents the temperature difference between the gas temperature and the mean temperature of the element l3 divided by the temperature difierence between the gas temperature and the wall temperature at the gas side of the tube. On this diagram the curve a-b represents the temperature gradient along the length of the element and the two shaded areas above and below the curve a-b, being equal in area, establish the point of mean temperature along the curve a-b.

Since a full appreciation of the significance of the heat transfer eiliciency m; is essential to a clear appreciation of our invention further explanation thereof is warranted. Expressed in another way, this symbol signifies the relation between the heat quantity transmitted through an element in a practical apparatus in which the temperature of the element by necessity increases along the length thereof and towards its outer end, and the heat transmitted by the same element under comparable conditions if the temperature throughout the element was constant and equal to the temperature at the base of the element where it is attached to the wall of the tube. The latter condition is, of course, impossible to obtain in an actual apparatus; but this criterion of efiiciency (us) can be effectively 7 used to establish the optimum maximum length of the elements.

The most convenient way of using this criterion is to set up a family of curves such as shown in Figure 11 by calculations employing the aforesaid formula. These curves may be plotted for any number of surface conductances, a values, for any given cross-sectional area of the elements. In Figure 11 the calculations involved in plotting the curves were predicated upon the cross sectional area of an element of 3 mm. diameter and in these calculations the heat transfer efficiency was mathematically determined from the 1;; formula for each assumed alpha value at assumed lengths of conductance paths. Thus for instance for the alpha value of 75 and a length of .01 meter the heat transfer efficiency is .989; for .02 meter the heat transfer efliciency is 963; for .03 meter it is .916; for .04 meter it is .863, etc.

If through the use of this 1 formula it is found that the emciency of the conductivity of the elements is at least sixty percent (60%) of theoretically complete (100%) heat transfer the dimenditions will permit.

In this connection it should be noted than an imaginary wall with no transfer losses whatsoever would provide complete heat transfer and one hundred percent (100%) efficiency of conductivity.

It should be understood that in the foregoing references to the length of the elements l3 we mean the length of the conductance path provided thereby to the nearest base surface. Where the elements are in the form of loops or rods projecting from the wall to which they are attached, the length of the conductance path is the distance measured along the axis of the element from its final point of contact with the base wall to the point farthest therefrom. Where the elements connect two tubes, as shown for instance in Figures- 3, 4 and 5, the length of the conductance path is one-half the distance (measured along the axis of the element) between the points of final contact between the elements and tubes.

The elements l3 should be of metal having a high coeflicient of thermal-conductivity, preferably at least 90 Kcal./hr./m.=/ C./m. Copper is ideal though other material such as aluminum, nickel, brass, zinc, steel and various alloys may be used.

The junction of each element l3 with its base surface, specifically the wall of the tube to which it is secured, should be at least as great as the cross sectional area of the element. The junctions may be effected by welding, brazing or any other way of securing a good heat transfer connection.

While the specific arrangement of the tubes and the heat transfer elements thereon may be varied, that shown in Figures 3 to 6 inclusive has proved especially satisfactory. As here shown each section of the convection surfaces consists of a plurality of banks or strata H of tubing with heat transfer elements mounted thereon and with the tubing arranged in serpentine fashion and all formed from one continuous length 8 of tubing. The serpentine coils comprising each bank are divided into three sets each of which has its ownheat transfer elements i3 thereon. Each set of coils with its heat transfer elements arranged thereon, thus may be considered a heat absorbing mat l5 and collectively these mats present a heat absorbing screen extending fully across the flue gas duct.

The elements I3 of each mat are preferably one continuous length of wire of a size lying within the prescribed limits of 3 and 50 mm. and chosen with respect to such considerations as necessary mechanical strength, gas temperatures and adequate provision for cleaning soot and dirt from the elements. In no event, however, should the cross sectional area of the elements exceed 50 mm. since wires or rods larger than this do not possess suiiicient advantage (in heat transfer) over the smaller diameter tubes used in boilers of the forced circulation type, as for instance, the La Mont.

We have found that the combined volume of all of the elements, i. e. the-total volume of the extended surface, should be between 5% and 25% of the volume of the zone or duct in which they are located, and that the total extended surface area provided by the elements and exposed to the gases should be not less than 50 mm. nor more than 200 mm. per cubic meter of that portion of the zone or duct containing the elements.

Also, we have found that the total exposed area of the heat transfer elements should be not less than three (3) nor more than sixteen (16) and preferably not less than five (5) nor more than ten (10) times as great as the projected area of the wall portion (exterior of tubes) adjacent to that part of the zone or duct contain ing the elements. It is to be understood that the projected area in the case of a tube is the diameter of the tube times the length thereof, and that the projected area thus considered should encompass the junctions of all of the elements to the wall and extend beyond the outermost or border junctions a distance one-half the spacing between adjacent junctions since for this area the transmission of heat by all of the elements into the wall will be the same.

In the application of the heat transfer elements [3 to the tubes, the wire from which the elements are formed is wound about the legs of the tube coils in elongated loops and then one side of every other loop is indented or deformed towards the median plane containing the axes of the tubes and over the tubes, and at the same time the remaining loops have their other sides similarly indented or deformed to bring about the staggered arrangement of the elements shown in Figure 5 and Figure 6, with all of the extended surface provided by the elements, of course, extending transversely or normal to the direction of gas flow. The extent of the indentations of the loop sides is such that the distance between the straight undeformed sides of the loops is equally divided by the indented portions.

To more clearly illustrate the application of our invention to a steam boiler, the essential dimensions and data for a typical installation are set out below. The example applies to the convection surfaces of a La Mont type boiler but not necessarily the one shown in Figures 1, 3 and 4, and contemplates the staggered arrangement of heat transfer elements like those shown in Figures 3-6 inclusive. The important data and dimensions of the installation chosen for illustration are as follows:

Volume of gas at C.=4350 normal mF/h.

Temperature of gas entering convection surfaces=1000 C.

Temperature of gas leaving convection surfaces=450 C.

Absolute mean temperature of the gas:

Steam pressure=25 atm. gauge Steam temperature (ts) :225 C. V0 (gas velocity) at 0 C.=2.1 m./sec.

The tubes are formed of standard steel boiler tubing and have an inside diameter of 12 mm. and an outside diameter of 16 mm. The size and amount of the tubing is determined with regard to the temperatures and heat release involved and must be such that the tubes at all times contain sufiicient water to maintain desirable flow characteristics. The water should exceed the steam appropriate formula given above (the first for staggered element arrangement) is 70% of this is 113 kg. cal./m. /C./h,

The value 6.3 is the S: dimension of the formula and is obtained as follows: the diameter of the tube, 16 mm., plus one diameter of the elements. viz, 3 mm., provides a center-to-center distance of 19 mm. between the straight undeformed sides of the loops formed by the heat transfer elements. Indenting the opposite sides of the loops in the manner described divides this distance of 19 mm. by 3 which makes the Si distance 6.3 mm. The 998 C. is the absolute mean temperature of the gas as noted hereinbefore.

Using the family of curves shown in Figure 11 to determine the length of the elements, 1. e. of the conductance paths afforded thereby, and an assumed heat transfer efliciency of 88% and an alpha value of 113 kg? cal./m'. /C./h. it will be found that; the length of the conductance paths provided by the looped elements l3 should be about mm.

The accuracy with which the curve is read in the above determination of the length of the conductance paths can be checked and verified by the formula as follows:

Since the conductance paths provided by the looped ends of the elements are of the same length as the paths provided by the straight portions of the elements from each tube to the mid- The optimum length of the conductance paths, I

of course, also determines the center-to-center distance between the tubes which in the illustration given comes to 60 mm.

By computing the surface area of the looped elements and deducting eleven percent (11%) for the approximate area thereof encompassed by the welded junctions of the loops to the tubes it will be seen that an extended heating surface of 117 m. per m? volume is obtained. This is well within the range heretofore prescribed, viz. 50 m. to 200 m. per m3.

The cross sectional dimensions of the flue duct are 600 x 1600 mm. and the mats, each 1600 mm. long, are arranged fiatwise edge-to-edge so as to dispose five mats in each bank across the flue duct. Ten banks of mats comprise the total convection surfaces with the vertical center-to-center distance between adjacent banks 25.2 mm. The total height of the entire mat assembly is thus 9x25.2+22 mm. (one mat thickness) =249 mm.

The volume of the gas pass or duct required for the entire mat assembly (the total heating surface) is 0.6X 1.6X0.249=0.239 mi.

The area of the heating surface contained within this volume is 117 0.239=27.9 m.

The dimensions and data given thus far apply to the gas side of the wall separating the two media, 1. e., the outside of the tubes. For the water side the following information is given.

The effective heating surface (inside of tubing in contact with water) is 6.0 m3. Inasmuch as the tubes contain a water-steamemulsion the surface conductance is at least 10,000 Kcal./m. C./h.

The mean temperature drop between the water-steam emulsion and the tube wall is 15.3 C., determined as follows:

4350 (1000 X 0.364-450 X 0.340) :916,000 KcaL/h.

where 0.364 is the average specific heat of the gas between 0 and 1000 C., and 0.340 is the average specific heat of the gas between 0 and 450 C.

The mean temperature drop in the tube wall= The corresponding difference at the point where the gas enters the convection surfaces consequentlywillbe and for the point where the gas leaves the convection surfaces it will be o X02.1 O. The wall temperature of the tube on the gas connected with the stack upstream.

side at the point of gas entry into the convection surfaces is thus and at the point the gas leaves the convection surfaces 225+10=235 C.

The temperature difference between the gas' and the elements at the hot gas inlet end of the convection surfaces is:

and at the cooler discharge end 0.88 (450-435) =189 C. 0.88 being in (the heat transfer efileiency) The logarithmic meanv temperature difference between the gas and the elements=374 C., which was calculated as follows:

To check the required heating surface, 1. e., effective surface of heat transfer elements whichisless thanused,viz. 219m.

In the illustration given it will be seen that the rate of heat transfer at the gas side (outside of tubes) has substantially approached themaXimum obtainable rate of heat transfer on the water side (inside of tubes) this being evidenced by a comparison of the products of the surface conductance times the heat transfer area at opposite sides of the wall.

The product of the surface conductance on the water side times the heating area is thus 19 times as great as the product on the gas side.

The significance of this conclusion will be seen when it is appreciated that if tubes alone were used, the surface conductance (alpha value) on the gas side would be decreased to about 67. This value, multiplied by the total surface area of the exterior of the tubes would result in a product of 535.5; and this compared to the product 60,000 would make the product on the water side 112 times greater than on the gas side.

In Figures 7 and 8 we have illustrated the application of this invention to an installation de-' signed to recover ordinarily lost heat and thus improve the efiiciency of an existing boiler which has been merely indicated and designated by the numeral It. In this embodiment of the invention a unit I1 is connected between the flue gas outlet of the boiler and the stack ll. To enable such connection a T-shaped duct 19 was sub- The unit I1 is mounted between the duct sections I! and 20 and comprises a flue gas passage 21 defined by two spaced water ducts 22 connected by two side plates 23. The water ducts 22 have an inlet header 24 connected to the upper ends thereof as at 25 and a similar outlet header (not shown) connected to the lower ends thereof as at 26. Within the fiue gas passage 2| and extending between the inside walls 21 of the water ducts are heat transfer elements 28.

- These elements extend transversely to the direction of the gas flow and consist of lengths of copper wire of a diameter ranging between two and eight millimeters wound about collector rods 80.

The elements are secured to the rods 30 and to the inside walls 21 of the water ducts in good heat transfer relation thereto by welding, brazing, or otherwise, and the combined volume of the heat transfer elements It constitutes between five and twenty-five percent (5% and 25%) of the total volume of the flue gas passage 2 I. Experience has shown that this ratio is best suited to the achievement of the purposes of our invention taking into account all the factors such as accessibility for cleaning, maintenance and adequate gas flow, etc., which govern the amount of heating elements that can be incorporated'in a gas pass of given volume. Obviously the ratio should be as high as possible consistent with these factors.

The combined area of the elements exposed to the gases is between three and sixteen times greater than the projected area of the walls 21 served by the elements, and preferably between five and ten times said projected area of the walls 21, this ratio being determined by the same considerations involved in the selection of the .in the T and elbow duct sections l9 and 20 respectively. With the removal of these covers cleaning brushes, soot blowers or other suitable I means may be employed to clean the accumulated soot and dirt from the elements".

Valves 32 are also preferably provided in the outlets of the duct sections 19 and 20 by which the gas fiow may be directed through the unit 11 or past it.

The results of tests conducted with a unit such as that disclosed in Figures 7 and 8 and extending over a period of four months are plotted on the graph of Figure 9. From this graph it is apparent that without the unit of this invention the boiler to which it was attached showed a decreasing efficiency with increasing load, as rep resented by curve A. At the prevailing load for the four month period, indicated by the line BB,

this boiler was less than fifty percent (50%) efficient. During the same period and under identical conditions the efiiciency of the same boiler with the unit [1 attached thereto and operating was eighty percent as shown by curve C. This represents a fuel saving of forty percent (40%) under the prevailing conditions, as indicated by curve D, the curves 0 and D, of course, being independent of one another.

The average temperature of the flue gases entering the unit I1 during these tests was 450 C. while the temperature of gases leaving the unit and entering the chimney or stack was only C.

As illustrated in Figure 12 the invention also improves the utilization of radiant heat as in a furnace. To this end the invention takes cognizance of the relatively great differential in surface conductance which obtains at opposite sides of a wall subjected at one side to the direct radiant heat rays emanating from the bed of burning fuel and at the other side to the air to be heated andin motion thereover.

In this embodiment of the invention a metal wall 33 is subjected to direct radiant heat rays emanating from a combustion zone 34. This wall coacts with an outer shell 35 and suitable end walls 36 (only one of which is indicated) to define an air duct or passage into which air to be heated is fed through an inlet 3'! and from which hot 'air is discharged through an outlet 38. Preferably the shell 35 is equipped with an insulating jacket 39.

Within the air passage thus defined are rows of serpentine heat transfer elements 40 attached to the wall 33 in heat transfer relation thereto by welding, brazing or otherwise but having no contact with the outer shell 35.

Through proper proportioning and spacing of these heat transfer elements 40 the rate of heat transfer between the air being heated and the heating surfaces can be increased to an extent which approaches the maximum obtainable rate of heat transfer between the radiant heat rays and the wall 33. In this case the heat rays are the medium of high surface conductance while the air to be heated is the medium of low surface conductance. Attention is directed to the fact that the situation which obtains here is not the same as where the wall separates media of substantially the same surface conductance.

Figures 13 and 14 illustrate the application of our invention to an oil prcheater. In this case the oil is the medium of low surface conductance, for the heating medium is condensing steam which enters the apparatus through a central duct 4|, flows along the length thereof to debouch into an annular passage 42 defined by the wall of the duct 4| and a tube 43. Both ends of this tube are closed and a discharge 44 leads from its exposed end portion through which the condensate leaves the apparatus.

The heating unit, consisting of the duct 4| and tube 43 assembled as described, projects endwise into a cylindrical shell 45 through one end wall 46 thereof, the opposite end of the shell 45 being closed by a plate 41.

The annular space between the tube 43 and the shell 45 provides for the passage of oil to be heated and which enters through an inlet 48 and leaves through an outlet 49. Heat transfer elements 50 are arranged within this annular space, being fixed to the duct 43 in heat transfer relaion thereto but clear of the shell 45. These elements 50 are preferably serpentine lengths of heavy wire arranged substantially radially about the duct 43 as shown in Figure 14. By virtue of the curved shape of the elements 50, as viewed in Figure 14, the space between them is substantially uniform throughout their length. This arrangement also facilitates cleaning of the elements which, of course, necessitates withdrawal of the heating assembly by removal of the end wall 46.

We have also devised simple formulae that will enable those fully skilled in the art, and even others less skilled, to determine with reasonable accuracy the borderlines within which the surface conductance (alpha value) of the medium of lower surface conductance and the total area of the heat transfer elements exposed thereto should lie in order to secure an efiicient apparatus of the kind described.

One of the factors of each of these formulae is the difference between the products of the surface conductance (alpha value) times the area on one side of the wall and the corresponding product on the other side of the wall. This factor is designated Y in the formulae. For each formula we have determined the limits between which the optimum value of this factor Y lies. The surface conductance of the medium having the lower alpha value has been designated :11 and the heat transfer surface area in contact therewith as A1; and for the medium with the higher surface conductance the correspondin designations are a2 and A2. The product of these latter values divided by the variable factor Y will give the a1 A1 product for the medium of the lower surface conductance, and with the factor Y given if any three of the other factors in the formula are known the value of the fifth factor can be easily determined.

-Apparatus designed and built according to these formulae while perhaps not perfect will have very high efficiency and small space requirements as contemplated by our invention.

The more important of these formulae follow:

For the convection surfaces of a steam boiler wherein the tubes contain a water and steam emulsion a2=approximately 10,000 kg. cal./m. /C./h., and the factor Y will vary between 10 and 40 and preferably between 16 and 24.

For water in motion as in the economizer tubes of a steam boiler, a2=approximately 5000 kg. cal./m. /C./h.. and the factor Y will vary between 10 and 50 and preferably between 20 and 30.

In such applications of the invention as in the preheating of oil where condensing steam is used as the heat source, a2=appr0ximately 10,000 kg. cal./m. /C./h., and the factor Y will vary between 8 and 24 and preferably between 10 and 15. I

With the information here given those fully skilled in the art will have little difliculty in determining the actual values applicable in each particular case, and in making corrections necessary to achieve still higher efficiency. For instance, in the case of water in motion, where the approximate alpha value was stated as 5000, if the actual alpha value is found to be 6000, the correct value for the factor Y can be established as follows:

Y.,= X Y (arbitrarily chosen as 20 from range given in formula) and substituting- Y,, (the desired correct= X20=24 factor) 5000 l ous limits herein prescribed apply to all embodiments of the invention-though not speciflcally stated inieach case, except where such limits are not applicable to a particular embodiment.

This'application is a continuation in part of the copending application Serial No. 23,726, filed April 28, 1948, now abandoned.

What we claim as our invention is:

1. A steam boiler having a flue gas passage, boiler fluid conducting tubes within the gas passage and a heat exchanger within the passage and including said tubes: characterized by the provision of: heat transfer elements joined to said tubes and collectively forming a heat absorbing screen extending across the entire flue gas passage, said elements being made of high conductivity material and having a substantially uniform cross section throughout their length of between 3 and 50 square millimeters and of a shape wherein the major transverse dimension does not exceed three times the minor transverse dimension, all of said elements being substantially parallel to one another and crosswise to the gas flow through the flue gas passage, and each element being of such length that the heat conductance path afforded thereby to the wall of the tube to which it is joined is not less than ten times the square root of the cross sectional area of the element nor longer than that at which the efliciency of its conductivity as determined by the formula a=the surface conductance (alpha value) of the gases flowing over the elements l=the conductance of the elements e=base natural logarithm (=2.7l8)

F=circumference of element in M l=cross sectional area of element in NP l=length of conductance path in M is less than seventy-five percent of theoretically complete heat transfer, for situations where the temperature to which the" elements are subjected is 600 C. or over and less than sixty percent of theoretically complete heat transfer for tempera tures under 600 0., the distribution of said heat transfer elements being substantially uniform across the entire flue gas passage, and the combined surface area of the elements being three to sixteen times as great as the projected area of that length of the tubes on which elements are mounted so that said elements constitute the primary heat exchange surfaces of the heat exchanger in contact with the flue gases.

2. The apparatus defined in claim 1 further characterized by the fact that the length of the characterized by the fact that the combined volume of the extended surface provided by said heat transfer elements is between flve and twentyfive percent of the total/volume of that portion of the flue gas passage containing said elements.

4. The apparatus defined in claim 1 further aaeabse i characterized by the'fact that the total surface area of the extended surface providedby the heat transfer elements exposed to the gases is not less than 50 square meters nor more than 200 square meters per cubic meter of that portion of the gas passage containing the elements.

5. Means for effecting heat exchange between two flowing media, comprising: means defining a passage through which one of the media flows; a plurality of ducts arranged in a group within said passage and through which the other medium flows; and heat transfer elements intimately joined to the exterior surface of the ducts and extending therefrom in a direction crosswise oi the flow through said passage, said elements having a conductivity of not lessthan kcal. per hour per square meter per meter thickness per degree centigrade difference in temperature, the junctions of said elements with the ducts being substantially uniformly spaced along the ducts in said group and each of said elements being of approximately round cross section and having a substantially uniform cross section throughout its length of between 3 and 50 square millimeters and the extended portion thereof having a length of not less than ten times the square root of its cross sectional area and not more -than that at which the efliciency of its conductivity, as determined by the formula and where a=the surface conductance (alpha value) of the gases flowing over the elements x=the conductance of the elements e=base natural logarithm (=2.718)

F=circumference of element in M f=cross sectional area of element in M l=length of conductance path in M is less than sixty percent of theoretically complete heat transfer.

6. The structure set forth in claim 5 further characterized by the fact that the length of the heat transfer elements is such that the product of the surface conductance of the medium flowing through the passage times the total surface area of the extended portions of the elements equals the product of the surface conductance of the medium flowing through the ducts times the area of the inside walls of the ducts in said group, after said latter product is divided by a factor lying between 8 and 40.

7. Means for effecting indirect heat exchange between two media confined to zones separated by a wall through which some heat transfer takes place characterized by the provision of: heat transfer elements intimately joined to said wall in heat transfer relation thereto and extending therefrom, whereby said elements are in direct heat transfer relation with the medium at said side of the wall and in indirect heat transfer relation with the medium at the other side of the wall, said elements being made of material having a conductivity of not less than 90 kcal. per hour per square meter per meter thickness per degree centigrade difference in temperature and the extended portion of each of said heat transfer elements having an approximately round and substantially uniform cross section throughout its length of between 3 and 50 square millimeters and having a length of not less than ten times the square root of its cross sectional area 17 and not more than that at which the eilicien y of its conductivity as determined by the formula m=the surface conductance (alpha value) of the gases flowing over the elements A=the conductance of the element e=base natural logarithm (=2.7l8)

F=circumference of element in M f=cross sectional area of element in M l=length of conductance path in M uXF xXf

where n of the zone in which they lie and the length thereof being such that the product of the surface conductance of the medium in direct. contact with said elements times the'total surface area of the extended portions of the elements equals the product of the surface conductance of the other medium times the area of the heat exchange surface exposed thereto after said latter product is divided by a factor lying between 10 and 30.

8. A heat exchange unit for use in steam boilers and the like, comprising: a pair of spaced substantially parallel water-conducting tubes; and heat transfer elements mounted 'on and connecting said tubes, said elements comprising a series of elongated wire loops embracing and in contact with both tubes and spaced apart along is millimeters throughout their lengths and of such cross sectional shape that the major transverse dimension does not exceed three times the minor transverse dimension, and said elements providing heat conductivity paths each of which is of a length not less than ten times the square root of the cross sectional area of the elements nor longer than that at which the efliciency of its conductivity as determined by the formula and where =the surface conductance (alpha value) of the gases flowing over the elements X=the conductance of the element e=base natural logarithm (=2.718).

F=circumference of element in M f=cross sectional area of element in M l=length of conductance path in M sage and a heat exchanger within the gas passage.

the length thereof, alternate ones of the loop sides at both sides of the medianplane containing the axes of the tubes and the major axis of each loop being substantially straight between their points of tangency with the curved surfaces of the tubes, the remaining loop sides having their portions in contact with the tubes wrapped partially around the curved surfaces of the tubes and being spaced inwardly of said first designated loop sides and substantialLv parallel thereto, the extent of said inward disposition of 'the second designated loop sides being such as to substantially equally divide'the center-tocenter distance between the first designated loop sides measured at right angles to said median plane; and means securing the loops to the tubes in intimate good heat exchange relation thereto.

9. The structure set forth in claim 8 further characterized by the fact that the wire of which said loops are formed has asub'stantially round cross section with a cross sectional area ofbetween 3 and 50 square millimeters and a conductivity of at least 90 kcal. per-hour per square meter per meter thickness per degree centi'grade diiferenceintemperature;

10. A heat exchanger comprising: means"defining two adjacent iiuid passages separated by a common wall; and a multiplicity'o'f heat transfer elements Joined to said wall 'in intimate heat transfer relation thereto and extending there from into and substantially entirely across the passage atthatside ofthewalhallofsaidheat transfer elements being of substantiallythe same uniform cross section of between 3 and 50 square and including said tubes, characterized by the provision of wire-like heat transfer elements of high conductivity metal joined to the tubes in intimate heat exchange relation thereto and extending therefrom with all of the elements crosswise to the gas flow and substantially uniformly spaced from one another throughout their lengths and also uniformly distributed across the flue gas passage, all' of said elements having a substantially uniform cross section throughout their lengths of between 3 and 50 square millimeters and the length of the extended portion of each element being such that the conductance path afforded thereby to the wall of the tube to which said path leads is not less than ten times the square root of its cross sectional area, nor longer than that at which the emciency of its conductivlty as determined by the formula 1 inl h=ax v i where nand where =the surface conductance (alpha value) or the gases flowing over th elements k=the conductance of the element e==base natural logarithm (=2.'l18) =circumference of element in M I=cross sectional area of element in M I=length of conductance path in M is less than sixty percent 60%) of theoretically 19 transfer elements is substantially round in cross section. v

13. A steam boiler having a flue gas passage,

boiler fluid conducting tubes within the gas passage and a heat exchanger within the gas passage, and including said tubes, characterized by the provision of: wire-like heat transfer elements of high conductivity metal joined to the tubes in intimate heat exchange relation thereto and extending therefrom with all of the elements crosswise to the gas flow and substantially uniformly spaced from one another throughout their lengths and also uniformly distributed across the flue gas passage, all of said elements having a substantially uniform cross section throughout their lengths of between 3 and 50 square millimeters and the length of the extended portion of each element being such that the conductance path afforded thereby tothe wall of the tube to which said path leads is not less than ten times the square root of its cross sectional area, nor longer than that at which the efllciency of its conductivity as determined by the formula 1 e"' 1 aXF X-m where n- W and where =the surface conductance (alpha value) of the gases flowing over the elements x=the conductance of the element a=base natural logarithm (=2.718)

F=circumference of element in M f=cross sectional area of element in M l=length of conductance path in M is less than sixty percent (60%) of theoretically complete heat transfer, the combined surface area of the extended .portions of all of said elements being between 3 and 16 times greater than the projected area of that length of the tubes on which elements are mounted so that said heat transfer elements constitute the primary heat exchange surface of the heat exchanger in contact with the flue gases, and the length of the heat transfer elements being such that the product of the surface conductance of the flue gases times the total surface area of all of the elements exposed to the flue gases equals the quotient derived from the division, by a factor lying between 16 and 24, of the product of the surface conductance of the boiler fluid times the area of the inside surface of that length of the tubes upon which the elements are mounted.

DAVlD DALIN.

GUSTAV V. HAGBY.

REFERENCES CITED The following references are of record in the ille of this patent:

UNI'I'ED STATES PATENTS Number Name Date Re. 10,543 Ryan Dec. 9, 1884 43,749 Martin Aug. 2, 1864 101,923 Rowe Apr. 12, 1870 166,461 -Houghton Aug. 10, 1875 213,157 Alvord Mar. 11, 1879 309,027 Burghardt Dec. 9, 1884 363,017 Stanton May 17, 1887 368,332 Gillespie Aug. 16, 1887 368,824 Stanton Aug. 23, 1887 369,470 Priest Sept. 6, 1887 373,576 Young Nov. 22, 1887 377,460 Lynn Feb. 7, 1888 384,461 Neil et al June 12, 1888 414,207 Gillet Nov. 5, 1889 Number 20 Name Date Gillet Nov. 12, 1889 Gillet Apr. 29, 1890 Stanton Nov. 28, 1893 Stanton Dec. 10, 1895 Mills Mar. 16, 1897 I Stuckel July 20, 1897 Pitkin Nov. 2, 1897 Stanton Mar. 29, 1898 Clarkson May 23, 1899 Wilkins et al Jan. 23, 1900 Nussbaum Jan. 23, 1900 Humphreys --Dec. 2, 1902. Lang Oct. 20, 1903 Nelson Feb. 2,1904 Cleveland Nov. 1, 1904 Hanscom et al Dec. 20, 1904 Pierce et al May 30, 1905 Zent May 8, 1906 Schiinleber June 18, 1907 Stanton July 2, 1907 Cole Sept. 10, 1907 Aylsworth et a1. Feb. 11, 1908 Fell Mar. 3, 1908 Roake Aug. 4, 1908 Reid July 6, 1909 Junkers Jan. 19, 1915 Rector Mar. 23, 1915 Shaw Aug. 3, 1915 Sterzing Aug. 13, 1918 Stuart Sept. 16, 1924 Hess Nov. 18, 1924 Doble Dec. 16, 1924 Hal-kin Apr. 14, 1925 La Mont July 14, 1925 Meaker July 21, 1925 Murray Aug. 4, 1925 Antisell Nov. 1'7, 1925 Clarkson Aug. 2, 1927 Solomiac Nov. 8, 1927 Aske Nov. 6, 1928 Stanclifle Nov. 20, 1928 Still June 11, 1929 Gay Aug. 2'7, 1929 Lattner Nov. 18, 1930 La Mont July 21, 1931 Hosbein Aug. 11, 1931 Marriott Oct. 6, 1931 Fountain May 31, 1932 La Mont May 31, 1932 Sauvan Sept. 13, 1932 McCausland Aug. 1, 1933 Murray et al. Oct. 10, 1933 Hatter Feb. 6, 1934 Simera'l et al. Oct. 23, 1934 Sorensen June 11, 1935 Hall Apr. 14, 1936 Truelsen Sept. 1, 1936 Wilson July 20, 1937 Murray, Jr. Aug. 3, 1937 Still Sept. 14, 1937 Murray, Jr. Sept. 21, 1937 Thompson et a1. Mar. 1, 1938 Poole Mar. 29, 1938 Anderson Apr. 26, 1938 Harris May 3, 1938 Cassidy et al. June 7, 1938 Finestone Nov. 1, 1938 Lee Feb. 7, 1939 Blum Oct. 15, 1940 Spanner Oct. 22, 1940 Wittmann Mar. 11, 1941 Jensen Apr. 1, 1941 Pascale June 10, 1941 (other references on following us Number Number 21 Name Date Spofiord Aug. 25, 1942 Kijgel Dec. 29, 1942 Barrett Aug. 24, 1943 Fassinger, Sr Feb. 29, 1944 Findley Nov. 7, 1944 Peters Feb. 26, 1946 Baver Apr. 15, 1947 FOREIGN PATENTS Country Date Great Britain May 10, 1900 Great Britain June 16, 1933 Great Britain July 26, 1933 France July 2, 1934 France Sept. 8, 1934 Great Britain July 8, 1935 Switzerland July 9, 1935 France Oct. 10, 1935 Germany Sept. 3, 1936 Sweden Mar. 2, 1937 France Mar. 14, 1938 22 Number Country Date 488,591 Great Britain July 11, 1938 231,124 Switzerland Feb. 29, 1944 OTHER REFERENCES Japan Society of Mechanical Engineers Transactions, vol. 2 (1936), pages 373-376.

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Certificate of Correction Patent No. 2,469,635. May 10, 1949.

DAVID DALIN ET AL. It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows:

Column 3, line 73, after the word considerin insert the; column 7, line 18, before the number 963 insert a decimal point; e 25, for temperature read temperatures; line 33, for than read that; column 8, line 28, for 50 mm? read 50 m line 29, for 200 mm? read 200 m.; colunm 11, line 40, before used insert that; column 12, line 28, for maintenance and read maintenance of; and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 18th day of October, A. D. 1949.

THOMAS F. MURPHY,

Assistant Commissioner of Patents. 

